Weddell seal description

The Weddell seal (Leptonychotes weddellii) is the most southerly breeding of all mammals (1)(4)(5), and one of the best known Antarctic seal species (2)(6). The spindle-shaped body is generally dark silver-grey in colour, lighter below, and mottled with black, grey and white blotches, which vary between individuals. During summer, the coat may fade to brownish. The muzzle, which is short and blunt, is usually pale in colour, and there are also lighter patches over the eyes. Pups are born with grey or light brown, woolly fur, darker along the back, which is replaced by the adult coat after about four to six weeks (2)(3)(4)(5)(7).

The male and female Weddell seal are similar in appearance, although the female may be slightly larger (2)(3)(4). For most of the year the plumpness of the body makes the head of this species appear disproportionately small, although body weight fluctuates widely between seasons (3)(4)(5). The front flippers are relatively short (3), and bear large black claws, which may aid in gripping the ice. There is a distinct tail (2). The Weddell seal is one of the noisiest seal species, and has an extensive underwater repertoire, with at least 34 different call types recorded, including whistles, buzzes, tweets and chirps. The calls show some regional variation, and are usually audible from some distance, and even from above the ice (2)(4)(6)(8).

Related species

Weddell seal biology

The Weddell seal is an accomplished diver, able to reach depths of over 600 metres, ranging out to 5 kilometres from a breathing hole in a single dive, and spending up to 82 minutes underwater before surfacing to breathe (1)(2)(3)(4). Underwater vision is excellent, and a nictitating membrane protects the eyes from salt water and blowing snow (2)(4). Holes in the ice, for breathing and hauling out, are created and maintained by cutting and sawing at the ice with the teeth (1)(2)(4)(5). The Weddell seal feeds mainly on fish, including the large Antarctic cod, as well as on squid, octopus and crustaceans(2)(3)(4)(5)(9), and has also been recorded to occasionally take penguins (9). Although adult Weddell seals are relatively safe from predators, some, particularly younger individuals, are occasionally taken by killer whales and leopard seals (2)(5)(6)(7).

The female Weddell seal gives birth on the ice, usually within a small colony of other females, but the species is only loosely social, and individuals remain well spaced out. Births take place between September and November, and a single pup is usually born, or occasionally twins. The female remains with the pup for the first few weeks, feeding it on milk that is exceptionally high in fat and protein, so that the pup gains almost two kilograms in weight each day. The female does not forage during this time, and loses weight (2)(3)(4)(5)(7). After the first two weeks, the female starts to take the pup into the water, where it learns to swim (2)(4)(6). Weaning occurs at about six to seven weeks. After the last pups have been weaned, the fast ice breaks up, the adults disperse, and the pups are left to fend for themselves (2)(4)(7).

Subadult Weddell seals tend to congregate in large groups near the edge of the ice, away from the breeding colony, while adult males set up underwater territories beneath the ice, typically around ice holes used by the females. Each territory may cover an area up to 50 metres by 400 metres, and is defended by loud calling and sometimes bloody fights (2)(3)(4)(5). Mating takes place in the water, after the pups have been weaned, around mid-December. However, the implantation of the new embryo does not occur until January or February, allowing the female to moult, feed, and recover from lactation before the next young develops, and ensuring that the young will be born the following spring, after a total gestation period of about 11 months (2)(4)(5)(7). The female Weddell seal reaches maturity from about three to six years, and the male at seven to eight years. Lifespan may be up to 25 years (4)(5)(6)(7).

Weddell seal range

The Weddell seal has a widespread and circumpolar distribution around Antarctica, as well as occurring on sub-Antarctic islands including the South Orkneys, South Shetlands and South Georgia. Occasional wandering individuals have also been recorded as far as Australia, New Zealand, Patagonia, the Juan Fernandez and Falkland islands, and Uruguay (1)(2)(3)(4)(5). There is some debate over whether or not the Weddell seal is migratory, with some individuals remaining in the most southerly parts of the range year-round, and others moving north in winter as the ice expands (1)(2)(3)(4).

Species with a similar range

Weddell seal habitat

The Weddell seal breeds on fast ice, right up to the shoreline of the Antarctic continent, and tends to feed in inshore waters (1)(3)(4)(6)(7). It can also be found offshore on pack ice, and on Antarctic islands that are seasonally ice-free (1)(3).

Weddell seal status

Weddell seal threats

Although no accurate population assessments exist, the Weddell seal is believed to be widespread and abundant, and is not considered globally threatened (1). The species has no natural land predators, so shows little fear of humans, but the inaccessibility of its habitat has so far protected it from large-scale commercial harvest. Many Weddell seals were taken in the past to feed people and dogs during Antarctic exploration and around research bases, but this practice has now stopped (1)(2)(3)(4)(6), and there are not currently thought to be any significant threats to the species (1)(7).

However, a number of potential future threats to the Weddell seal do exist, including the spread of diseases such as canine distemper virus, an increase in tourism in the region, and pressure on fish and krill stocks from commercial fisheries (1)(6). The potential effects of global climate change are currently unknown, but the loss of large amounts of Antarctic sea ice could have a potentially significant effect on the species (1). In addition, if any commercial harvest was to begin, the Weddell seal would be particularly vulnerable due to its tendency to congregate in traditional areas of stable ice during the summer (4).

Weddell seal conservation

As with all Antarctic seal species, the Weddell seal is protected under the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals, which would control and regulate any potential future harvest of the species (1)(7)(10)(11). A plan by Norwegian scientists in 2000-2001 to kill 20 Weddell seals, as well as 60 other seals, for research purposes, was cancelled after the plan was opposed by environmental organisations and eventually rejected by the Norwegian government (6). The proximity of Antarctic research bases to some Weddell seal populations has caused disturbance to the seals in the past, and in 1998 the Environmental Protection Protocol to the Antarctic Treaty was approved, which put into place various environmental protection measures, such as the banning of mining and oil drilling in Antarctica, and the banning of refuse disposal and the use of pesticides in the region (6). Although these measures go some way towards protecting the Weddell seal and its habitat, significant action may also needed on overfishing and climate change if the future of this Antarctic mammal is to remain secure.

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Glossary

Crustaceans

Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Fast ice

Sea ice that forms along and remains attached to the shore, and, unlike floating pack ice, is typically immobile.

Gestation

The state of being pregnant; the period from conception to birth.

Nictitating membrane

A thin, tough, transparent or translucent membrane, or ‘inner eyelid’, found in various species of birds, reptiles, amphibians and fish, but less common in mammals. The nictitating membrane can be drawn across the eye to protect it from damage, or to moisten the eye whilst maintaining vision.

Pack ice

Sea ice that floats on the surface of the water. Often formed from large pieces of ice that consolidate into a single ice mass, pack ice typically moves with currents, tides and wind.

Territory

An area occupied and defended by an animal, a pair of animals or a colony.

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